1. Field of the Invention
The present invention relates to a transmission modulation apparatus, and more particularly, a transmission modulation apparatus to which a polar modulation scheme is applied.
2. Description of the Related Art
In the design of a transmission modulation apparatus of the related art, there is typically a trade off relationship between efficiency and linearity. However, recently, a technique has been proposed that is capable of realizing both high efficiency and linearity in the transmission modulation apparatus as a result of using polar modulation.
FIG. 1 is a block diagram showing a configuration example of a transmission modulation apparatus to which polar modulation is applied.
In FIG. 1, transmission modulation apparatus 10 is provided with high-frequency power amplifier 11 that amplifies phase-modulated high-frequency signals, and power supply voltage control section 12 that controls a power supply voltage of high-frequency power amplifier 11 based on a baseband amplitude signal.
Phase-modulated high-frequency signal 13 is inputted to high-frequency power amplifier 11, and baseband amplitude signal (for example, √(I2+Q2)) 14 generated from a baseband signal by amplitude/phase data generating section (not shown) is inputted to power supply voltage control section 12.
Phase-modulated high-frequency signal 13 is obtained by first generating a phase component (for example, an angle between a modulation symbol and an I axis) of the baseband signal by the amplitude/phase data generating section (not shown), and then modulating a carrier frequency signal using the phase component. Further, the power supply voltage formed by power supply voltage control section 12 is supplied to high-frequency power amplifier 11.
By this means, transmission output signal 15 obtained by amplifying a signal in which a power supply voltage value is multiplied by phase modulation high-frequency signal 13, by the amount corresponding to gain of high-frequency power amplifier 11, is outputted from high-frequency power amplifier 11. Transmission output signal 15 is transmitted from an antenna (not shown).
By using a polar modulation scheme in this way, it is possible to take phase-modulated high-frequency signal 13 inputted to high-frequency power amplifier 11 as a constant envelope signal which does not have a fluctuation component in an amplitude direction, so that it is possible to use a high-efficiency non-linear amplifier as high-frequency power amplifier 11.
However, with this kind of the polar modulation scheme, it is required that there is a proportional relationship between a voltage value of baseband amplitude signal 14 and an output voltage of high-frequency power amplifier 11 (typically obtained by converting the output power to a voltage applied to 50Ω).
As a device used for high-frequency power amplifier 11, it is often the case that an HBT (Hetro-junction Bipolar Transistors) device that gives higher gain than an FET device and can be made small easily is used. However, the HBT device has a specific parameter, which is referred to as an offset voltage, between the power supply voltage value and the output voltage.
FIG. 2 shows a relationship between the power supply voltage value and the output voltage when the HBT device is used as high-frequency power amplifier 11. As shown in FIG. 2, it can be understood that the power supply voltage and the output voltage change linearly, but this line does not pass through the origin, and therefore there is no proportional relationship. An offset voltage is required when the HBT device is used as high-frequency power amplifier 11. The offset voltage is a power supply voltage value when the output rises. In FIG. 2, the relationship between the power supply voltage value and the output voltage is made collinear approximation, and the intersection point of this line and the x axis is defined as an offset voltage.
In the polar modulation scheme, power supply voltage control section 12 avoids distortion by adding the above-described offset voltage to baseband amplitude signal 14 and performing correction so that there is a proportional relationship between the voltage value and the output voltage of baseband amplitude signal 14. In FIG. 2, when the slope changes, only the gain of the output voltage with respect to the power supply voltage value changes, and therefore distortion does not occur.
However, the relationship between the power supply voltage and the output voltage of the high-frequency power amplifier changes according to the input power and characteristic variations of the high-frequency power amplifier, and therefore the offset voltage also changes along with this. In particular, a region where the power supply voltage value is low is strongly susceptible to the influence of leakage of the high-frequency power amplifier. In FIG. 2, non-linear portions of the output voltage indicate the influence of leakage when the power supply voltage is low. The leakage amount of the high-frequency power amplifier is decided by parasitic capacitance of the devices, or the like, and therefore is susceptible to the influence of characteristic variations, and non-linear factors increase.
A region where the power supply voltage value is low corresponds to the case where a voltage value of the baseband amplitude signal is small. In the region, the proportion of the offset voltage increases, and therefore the characteristic is easily influenced. Because of this, there is a problem that, if the relationship between the power supply voltage and the output voltage is made collinear approximation and correction is performed using the offset voltage obtained from the intersection point of the line and an x axis, it is not possible to obtain sufficient effects.
Therefore, Patent Document 1 (Japanese Patent Application Laid-Open No. 2004-7434) discloses a technique of compensating linearity between a voltage value and an output voltage of a baseband amplitude signal by monitoring an output of a high-frequency power amplifier.
FIG. 3 is a block diagram showing a configuration example of a transmission modulation apparatus of the related art dealing with characteristic variations of a high-frequency power amplifier by forming an amplitude loop. Components that are the same as those in FIG. 1 will be assigned the same reference numerals. As shown in FIG. 3, amplitude loop 16 is always formed for monitoring the output of high-frequency power amplifier 11 and setting power supply voltage values corresponding to characteristic variations of the high-frequency power amplifier, and compensates the linearity between the voltage value and the output voltage of the baseband amplitude signal.
However, with such a transmission modulation apparatus of the related art, the amplitude loop is always formed as shown in FIG. 3, and therefore the circuit configuring the amplitude loop is required to have high linearity and wide band characteristics. As a result, there is a problem that a circuit scale and cost increase, and loop operation cannot be followed when the modulation speed becomes a high speed.
Further, when the power supply voltage is low as shown in FIG. 2, non-linear portions occur due to the influence of leakage of the high-frequency power amplifier. The collinear approximation is not enough for the non-linear portions, and therefore a method of performing compensation by storing all characteristics and reading out the characteristics is adopted. This is the same on the phase side. It goes without saying that an increase in the amount of used memory leads to an increase in cost. In addition, it is necessary to always execute operation by the amplitude loop as described above at non-linear portions where the power supply voltage is low. If appropriate correction is not carried out at non-linear portions, the characteristics shift from the ideal characteristics, and the shifted portion appears as a distortion component. In this case, the problem is influence on adjacent channels, and the influence is strictly defined by the specification.